The present invention relates to a method of monitoring protective gear for an electric power grid, where a voltage signal is derived from the line voltage and a current signal is derived from the line current. The present invention also relates to a protective gear device operating according to this method. The term xe2x80x9cprotective gear devicexe2x80x9d is understood here to refer to a remote protective gear in particular.
A remote protective gear or a protective relay is generally used in a high-voltage grid or a medium-voltage grid of an electric power distribution system to detect defects in a protective object assigned to this remote protective gear, namely a protective section or line section. To do so, the remote protective gear detects current and voltage values for the line or section to be protected within the total network and calculates the corresponding impedance from it.
When a fault occurs, e.g., in the form of a short circuit within the line section monitored, the remote protective gear triggers a tripping signal, which activates a circuit-breaker which in turn isolates the corresponding line section from the total network. The remote protective gear is used to localize the fault by determining the distance between the fault and the corresponding measurement station on the basis of the impedance thus determined, where the distance from the fault decreases with a decrease in impedance.
The remote protective gear receives the current and voltage values over appropriate measuring transducers whose secondary current and voltage measuring circuits supply a current signal proportional to the operating current, i.e., the line current of the line to be protected or a voltage signal proportional to the line voltage. A protective or monitoring device, e.g., in the form of a computer or microprocessor, forms the impedance (Z) from the quotient of the voltage signal and the current signal, comparing this with a setpoint (Zsoll). If the measuring circuit voltage drops in the case of a short circuit or a line break, and thus the impedance also drops, the line segment monitored is shut down when the tripping criterion (Z less than Zsoll) is met.
Occurrence of a fault in the measuring circuit, in particular failure of the voltage signal in the voltage measuring circuit (measuring circuit voltage failure) is problematical. Such a fault may occur, for example, due to a short circuit or a line break on the secondary side of the voltage transformer. Occurrence of such a fault leads to an unwanted shutdown of the object protected when the operating current is flowing; in other words, the line or line segment is shut down, because the tripping criterion is also met. In this case, the monitoring device may not differentiate between such a fault in the voltage measuring circuit and a line fault resembling a short circuit in the immediate vicinity of the remote protective gear, because in both cases the measuring circuit voltage is below a setpoint, or in the extreme case it may even be zero. In any case, the impedance derived from it is lower than the setpoint. For fault-free functioning of the remote protective gear, it is therefore necessary to guarantee an unambiguous differentiation between a line fault and a fault in the measuring circuit regardless of the type of network of the object to be protected.
If a protective switch is provided for the voltage transformer in the voltage measuring circuit, its circuit state can be used for analysis, e.g., by way of a binary input of the remote protective gear. A corresponding shutdown signal of the voltage transformer protective switch characterizes a short circuit in the voltage measuring circuit and can therefore be used to block the remote protective function. This prevents unwanted operation of the remote protective gear when there is a fault similar to a short circuit in the voltage measuring circuit. One disadvantage of this is that only a portion of the possible faults in the measurement circuit can be detected with such a method. For example, a line break in the measurement circuit cannot be detected.
With a three-phase grounded network, there is the possibility of monitoring the zero voltage and zero current. A suitable monitoring device would then block the remote protective function when an adjustable zero voltage limit value is exceeded and no zero current occurs at the same time. One disadvantage of such a method is its restricted applicability, especially since this principle is unfavorable in single-phase networks and in networks with an ungrounded neutral point.
In addition, this method has a faulty response to three-pole interruptions in the voltage measuring circuit because the remote protective gear is unintentionally tripped due to the lack of a zero voltage in this case. In a digital remote protective gear, blocking of the protective function in a measuring circuit voltage failure is often automatically associated with activation of an emergency protective function, preferably a time-overcurrent protective function.
An object of the present invention is to provide a method of operating a protective gear with which reliable detection of a fault in the measurement circuit of the protective gear is guaranteed, in particular a measuring circuit voltage failure in the voltage measuring circuit of a remote protective gear. With a protective gear that operates according to this method, unwanted operation due to a measuring circuit voltage failure should be reliably prevented.
According to the present invention, a logic operation is performed on a detected voltage drop and a detected current surge. By appropriate analysis of the current and voltage signals and performing a logic operation on them, this ensures reliable differentiation between a line fault and a failure in the measurement circuit, in particular in the voltage measuring circuit of a remote protective gear. On the other hand, this reliably prevents a tripping time delay in the case of a fault in the monitored line segment, in particular a line short circuit.
When a current surge is detected, this indicates a line fault or a network fault. The logic operation of a first detection signal characterizing the voltage drop with a second detection signal characterizing the current surge does not result in blockage of the protective function of the protective gear. This reliably avoids failure of the protective gear to trip, so the protective gear can still shut down a possible short circuit in the network.
By monitoring the voltage signal and thus the line voltage to detect when it falls below a predefinable threshold, a tripping signal or an output signal for blocking the protective gear is optionally delivered when no current surge has been detected and the current signal has not fallen below a lower current limit and has not exceeded an upper current limit. Thus, unwanted operation of the protective gear is also safely prevented.
To do so, the second detection signal derived from the criterion of the current surge is extended in time and inverted, and a logic operation is performed between the resulting signal and the first detection signal derived from the criterion of the voltage drop. At the same time, the current signal is compared with a lower-threshold value, i.e., with a lower current limit, and a logic operation is performed between a third detection signal derived from this comparison and the second detection signal. In addition, the current signal is advantageously compared with an upper threshold value, i.e., an upper current limit, and a logic operation is performed between a fourth detection signal derived from this comparison and the first and second detection signals. Thus, line breaks in the measurement circuit, in particular in the voltage measuring circuit, are also detected, and unwanted tripping of the protective function is prevented.
If the value of the monitored current signal is outside predefinable current limits, generation of an output signal and thus blockage of the protective function in general are prevented. Consequently, the protective function is always in effect when large currents in the short-circuit current range occur. Threshold monitoring with generation of corresponding detection signals thus prevents unwanted blocking of the protective function in the event of a network short circuit. In addition, the protective function and thus the remote protection are generally in effect at very low current signal levels and are neither blocked nor deactivated by the monitoring. The de-energized condition of the system, i.e., of the line, is also taken in account.
In an one example embodiment of the present invention, the output signal generated on the basis of the detection signals is used to block the protective gear or the remote protective function as well as to activate an emergency protective function. The malfunction or trouble in the protective gear detected on the basis of this criterion is then advantageously reported during a predefinable period of time. As soon as the voltage signal exceeds a threshold value assigned to it or the current signal drops below a threshold value assigned to it, the output signal generated on the basis of the detection signals to block the protective gear is deactivated again. Thus, the monitoring resumes its initial status when the voltage recovery is detected or the value of the current signal drops below a minimum current limit.
This method is suitable for a single-phase network, a two-phase network or a three-phase network with a grounded neutral point. The current surge is detected as a function of phase or based on phase. To monitor a three-phase network with an insulated or compensated neutral point, the neutral displacement voltage is preferably also included in the logic operation. To do so, another detection signal is generated on the basis of a comparison of the neutral displacement voltage with a threshold value and then gated with at least the first and second detection signals. Monitoring is in effect then only when the adjustable threshold is not exceeded by the detected instantaneous displacement voltage.
According to the present invention, the protective gear includes a monitoring device connected to a measurement circuit for converting the line current or operating current and the network voltage into proportional current and voltage signals and generating a tripping criterion or an output criterion from the voltage signal and the current signal for blocking the protective gear by using a number of logic elements. To do so, a first AND element is provided for gating a first detection signal derived from the current signal with a second detection signal generated from the voltage signal. A (fourth) detection signal derived from a deviation in the current signal from an upper threshold value is preferably sent to the first AND element over a comparator or flip-flop element.
In addition, a second AND element may be provided upstream from the link between the first detection signal and a (third) detection signal derived from a deviation in the current signal from a lower threshold value. A timer which is in turn provided upstream from the second AND element is also used to provide an expansion of the second detection signal and thus the current surge in time. After detection of a current surge, a timer module is started in the monitoring device by the timer, preventing output of an output signal for blocking the protective gear until a predetermined period of time has elapsed. This period of time may be longer than the longest grading time of the protective gear in order to reliably block the monitoring device in the event of a short circuit.
To maintain the output criterion for blocking the protective gear even when all other criteria for blocking the protective gear except the current surge criterion are met, the output of the first AND element is connected to one of its inputs via an OR element which in turn receives at the its input the second detection signal for a current surge. This ensures that the output signal also remains active with timing of the current surge as long as a measurement circuit fault is still detected. The output signal is preferably sent over a second timing element to generate an alarm signal indicating the respective fault.
The advantages achieved with the present invention include in particular the fact that reliable monitoring of protective gear, in particular remote protective gear, is possible without any restriction on fast response time due to a logic operation performed on a detected voltage drop and a detected current surge. This monitoring permits reliable differentiation between a system fault and a line fault, i.e., a fault in the protective object monitored, and a fault in the measurement circuit of the protective gear, in particular because of a short circuit or a line break in the voltage measuring circuit. This reliably prevents a delay in trip time in the event of a short circuit in the network or line. The method described here and the corresponding protective gear are thus suitable both for single-phase and multi-phase network systems, preventing unwanted operation of the protective gear as well as failure to trip. An example application is for remote protection, e.g.. in railway technology.